Our broad, long-term objective is to understand the neural mechanisms underlying age-related changes in the perception of speech in noise. To this end, it is necessary that audiological, psychoacoustical, and neurophysiological performance data in human subjects be obtained. These will contribute to our understanding of the psychophysical and neurological bases of age-related changes in speech perception in noise and the appropriateness of various species as animal models of presbycusis. In this project, 152 male and female subjects will be recruited to form four groups: young adults with normal audiometric hearing, aging adults with normal audiometric hearing, young adults with mild losses, and aging adults with mild losses. Each subject will participate in three series of experiments which will examine their performance on audiological, psychoacoustical, and neurophysiological measures. First, age-related changes on measures of speech reception in noise and other audiological tests will be determined. Second, temporal and frequency resolution will be measured psychoacoustically. Third, temporal resolution and frequency resolution will be measured neurophysiologically. All experiments will be conducted both in quiet and in a broadband masking noise. High-pass masking noise, will be used to investigate whether differences in performance between young and aging groups is due, in part, to unmatched high-frequency thresholds. The rationale for all of these experiments is to correlate age-related changes in speech perception in noise and audiological measures with age-related changes in the perception and processing of other complex signals which presumably underlie speech perception in noise. Completion by these same subjects of the same or similar experiments in the startle inhibition paradigms of Project 2 provides linkage between Projects 1 and 3. Completion of these same experiments by animals in Project 3 will allow for testing of C57B1/6J and CBA/J mice and the Fischer 344 rat as appropriate animal models for presbycusis. Audiological, including speech perception in noise, experiments, will take about 5 hours/subject, psychoacoustic experiments about 20 hours/subject, and neurophysiological experiments, 10 hours/subject for a total of 35 hours/subject. Each subject will be seen for approximately 3 hours/week for a total of 35 hours/subject. Each subject will be seen for approximately 3 hours/week for a total of 12 weeks. Ten subjects can be accommodated in one 12 week block and data will be collected for three 12 week blocks during each year for a grand total of approximately 30 subjects each year. Underlying relationships between audiological tests, speech perception in noise and psychoacoustical and neurophysiological measures of temporal and frequency resolution will be quantified using correlational and multivariate techniques.